This invention is related to apparatus and a method for determining the amount of gases, such as oxygen, nitrogen, carbon dioxide and carbon monoxide, dissolved in liquids such as water or samples of blood. More specifically, it relates to apparatus which, unlike most conventional apparatus used for such determinations, does not employ liquid mercury, which is toxic and represents a health hazard. When mercury spills occur in workplaces such as laboratories, mercury vapor creates an inhalation hazard at room temperature because of the low concentrations of mercury vapor required to cause ill effects.